The spectrum of human diseases caused by HSV-1 are a direct consequence of the dual life cycle of the virus. Thus, HSV-1 undergoes productive infection of epithelial cells at the site of primary infection and establishes latency in the nuclei of neurons that innervate this site. Life-long latent infections are characterized by periodic reactivation, virus shedding, and the recurrence of clinical disease. Although little is known about the molecular mechanisms that regulate the switch between latency and reactivation, viral DNA synthesis is a pivotal event that leads to the reactivation of the latent viral genome. Therefore, understanding the mechanisms that control HSV-1 DNA replication during productive infection and latency is an important public health priority. This proposal will attempt to identify functional differences between the two distinct types of origin of HSV-1 DNA replication, oriL and oriS, the sites at which viral DNA replication begins. Point mutations will be introduced into regions of oriL and oriS presumed to be important for origin function. The mutant origins will be compared to wild type origins using an in vitro origin-dependent DNA replication assay in neural and nonneural cells. Mutations that produce informative phenotypes will be introduced into the HSV-1 genome and viral replication will be studied in vitro in cells of neural and nonneural lineage. Additionally, mutant viruses will be further characterized in mice using a murine ocular model during acute infection and reactivation from latency to identify functional in vivo differences between the two types of origin. Since the viral genome is packaged into chromatin during neuronal latent infection, the nucleosome phasing at oriL and oriS will be examined to determine whether either origin is located in a region of open chromatin suggesting the preferential usage of one origin over the other during viral reactivation.